In the past, mobile communication systems have primarily used circuit-switched networks to provide voice services and low speed data services and packet-switched networks to provide high-speed data services. Circuit-switched networks allocate a dedicated channel for each voice or data call. In packet-switched networks, data is transmitted in packets over shared network resources. In general, packet-switched networks typically provide increased bandwidth efficiency as compared to circuit-switched network, while circuit-switched networks typically provide higher quality of service guarantees. The third generation (3G) standard integrated packet-switched data networks with circuit-switched voice networks to provide both voice and data services.
The fourth generation (4G) standard under development and known as Long Term Evolution (LTE) is a packet-switched network that does not have inherent support for voice services. A number of proposals are under consideration for providing voice communications in LTE networks. However, it is uncertain at this point whether the initial roll-out of LTE systems will include support for voice communications. If the initial roll-out does not provide support for voice communications, the service providers can leverage existing circuit-switched networks to provide voice services. Even if the early LTE systems support voice communications, the service providers will likely phase in LTE systems gradually and leverage existing 3G networks to provide service in areas where LTE networks do not provide coverage. Therefore, interworking protocols are needed to enable interworking between LTE and existing circuit-switched networks.
Several proposals are being considered to enable interworking between 3G and LTE networks to allow service providers to leverage existing networks and gradually phase in LTE networks. One approach to interworking is known as Single Radio Voice Call Continuity (SRVCC). The SRVCC approach allows an LTE voice call to be handed over to a 3G network when LTE coverage is not available. The SRVCC approach is described in 3GPP TS.23.216. Another interworking approach is known as Circuit-Switched Fallback (CSFB). CSFB is an interworking mechanism that allows service providers to use existing circuit-switched networks to provide voice services to LTE users. A mobile user can register with the circuit-switched network after attaching to the LTE network. For voice communications, the user is redirected from the LTE network to a 3G network providing voice services.
In the 3G network, a congestion control mechanism may be used to control congestion. The objective of the congestion control mechanism is to reduce the number of mobile-terminated and mobile-originated calls when the network is overloaded. The congestion control mechanism uses a persistence parameter broadcast to the mobile terminals over a downlink broadcast channel. The persistence parameter determines the probability that the mobile terminal will attempt to access the network. Typically, the mobile terminal performs a random check with the persistence parameter. If the mobile terminal passes the random check, the mobile terminal is allowed to access the network. Because the persistence parameter is transmitted in the 3G network, a mobile terminal operating in the LTE network may not receive the persistence parameters. Thus, the mobile terminals in the LTE network will not know when the 3G network is congested.
It has been proposed to broadcast the persistence parameter over the LTE broadcast channel. When the operator detects that there is an overload in the 3G system, the operator can update the persistence parameter in LTE so that the number of circuit-switched fallback attempts is reduced or stopped for a time period. Similar to 3G systems, the mobile terminal will read the persistence parameter and perform a random check before attempting circuit-switched fallback. Thus, signaling loads caused by failed circuit-switched fallback attempts on the LTE radio interface are avoided.
Another proposal is to convey congestion information to the mobile terminal over the LTE broadcast channel after the mobile terminal attempts circuit-switched fallback. When the mobile terminal attempts circuit-switched fallback, the mobile terminal sends a circuit-switched message to the mobile switching center in the 3G network. The interworking function may either accept or reject the signaling message. In either case, the interworking function can indicate the congestion status in a response to the signaling message.
The proposed solutions for congestion management are not entirely satisfactory. The problem with the first proposal is that there is no standard mechanism for making the congestion status of the 3G network known in the LTE network. Although vendors may implement proprietary protocols within the operation and management systems to convey congestion information from the 3G network to the LTE network, the vendor equipment will, in such case, not be interoperable with other vendor equipment. Further, the dynamic nature of the congestion status means that the proprietary system needs to update the congestion information dynamically. Conventional operation and management systems are not normally designed to provide dynamic functionality.
The problem with the second approach is that the congestion status of the 3G network is known only after the mobile terminal attempts circuit-switched fallback. Unnecessary signaling over the LTE network will result if a circuit-switched fallback attempt fails due to congestion in the 3G network.
Both of the proposed approaches contemplate using System Information Block 8 to broadcast congestion information. System Information Block 8 is a common channel information element. Continuously broadcasting congestion information over the broadcast channel in the LTE network would consume signaling bandwidth that could be used for other purposes.
Accordingly, there is a need for a mechanism to convey congestion information from the 3G network to mobile terminals in the LTE network.